The present invention relates to a calibration system for gas analyzers, particularly for apparatus measuring atmospheric pollution.
For studying atmospheric pollution (gaseous pollutants, fumes, particles), measuring networks are used, constituted by stations distributed over a given territory. In these stations, gas analyzers permanently measure the contents of the pollutants studied. In order to obtain reliable results of analysis, it is necessary periodically to calibrate the gas analyzers with the gaseous pollutants which are monitored. Furthermore, it is also necessary to ensure the physical intercalibration of the analyzing apparatus, i.e. to ensure that the pollutant and air standards and the sampled air are taken in the measuring station under the same conditions.
Calibration systems are already known which use liquefied pollutant gases contained in permeation tubes generally made of "Teflon". One of the walls of this tube is generally scavenged by "zero air", i.e. air containing no pollutant, or another gas such as nitrogen, at a constant rate of flow, the zero air or nitrogen passing around the permeation tube. Because the pressure of the liquid pollutant contained within the permeation tube is equal to the saturating vapour pressure, and because the partial pressure of the pollutant in the zero air or the nitrogen which passes about the permeation tube is zero, the existence of a pollutant pressure gradient on either side of the wall of the tube is created, such gradient causing the pollutant(s) to diffuse through the wall of the tube from the interior of the tube towards the exterior of the tube. This pressure gradient remaining constant, the rate of diffusion through the tube remains constant.
Such a system incorporating a liquid permeation tube presents a certain number of drawbacks. In the first place, the pressure of the pollutant gas increases exponentially with the temperature, this being detrimental to maintaining a constant concentration. Furthermore, the saturating vapour pressure inside the liquid permeation tube remains fixed, which does not enable the rate of permeation to be varied. In addition, these systems effect calibration only with the gases which are liquefiable in conventional permeation tubes, such as SO.sub.2, NO.sub.2, H.sub.2 S. The temperature of permeation at which the tube containing the liquefied pollutant gas is maintained, is relatively low in order not to create too high pressures inside the tube. By reason of these relatively low temperatures, the variation with respect to ambient temperature is relatively small, the consequence of which is that the temperature regulating system does not always operate under good conditions.